1. Field of the Invention
The present invention relates to methods and apparatuses for monitoring differentials between signals, and, in particular, to methods and apparatuses for improving the immunity of such devices to spurious or erroneous difference indications.
2. Description of the Related Art
It is known to use differential devices to compare two measured values of some real event or process, to detect problems, faults, or other occurrences that are indicated by a difference between the two measured values. For example, in a generator having a winding through which a given current flows, if there is no intended tap or loss of current between points A (where current enters the winding) and B (where current leaves the winding) of the current path, then current I.sub.A should be approximately equal to I.sub.B. A problem such as an insulation failure in the windings causing a short circuit or related problem between points A and B, can draw away current so that I.sub.A is not equal to I.sub.B. Thus, the existence of a difference between I.sub.A and I.sub.B can be used to indicate a potential problem, such as a loss of current in the windings of the generator.
It is known to monitor and compare these currents to ensure that the currents are approximately equal at all times. When I.sub.A and I.sub.B differ significantly, this may indicate that some problem has occurred between points A and B. This difference indication may be used to determine a resonse, e.g. setting off alarms to warn operators of the problem, or shutting off the generator. The difference between I.sub.A and I.sub.B may be a simple mathematical difference which is compared to a threshold value to determine if the currents differ significantly at any given point in time. Other conventional methods involve performing a fourier transform at each sampling interval on one cycle's worth of the samples (e.g. 12 samples) of I.sub.A and I.sub.B and making a more complicated comparison which indicates whether there is a significant difference between I.sub.A and I.sub.B for each sampling interval. A fault detection routine is typically configured to indicate a fault when, for example, seven such consecutive differences are indicated.
However, it is possible for a difference to be caused by spurious data or other random noise rather than by an actual fault or other problem. For example, the measured magnitude of I.sub.A may increase significantly for a brief period of time due to a voltage spike or other spurious data error. In this case I.sub.A and I.sub.B may differ significantly, thereby erroneously causing a fault to be indicated by a fault detection routine comparing the two currents. In this application such a spurious data reading may also be referred to as a "glitch" or as "noise." When such a glitch causes a fault to be indicated, it is possible that alarms may be set or systems shut down, causing great expense, inconvenience, or other problems. When a fourier transform is used, for example, a very large spurious reading of I.sub.A may affect the fourier transform calculation for an entire cycle (e.g. 12 samples), since the fourier transform utilizes the current sampled value of I.sub.A and I.sub.B and the previous 11 samples of I.sub.A and I.sub.B. Thus a single spurious reading of I.sub.A may affect the fourier transform output for many consecutive samples, erroneously causing a fault to be indicated.
It is accordingly an object of this invention to overcome the disadvantages and drawbacks of the known art and to provide a computer-implemented method and apparatus for differential fault indication that is more secure in terms of being less likely to erroneously indicate a fault.
It is a further object of this invention to provide such a differential fault indicator that is more tolerant of spurious data such as noise that momentarily and falsely indicate a fault, while still providing accurate and sensitive indications of actual faults when they occur.
Further objects and advantages of this invention will become apparent from the detailed description of a preferred embodiment which follows.